Power MOS devices can feature integrated circuitry suitable to protect the power device against excessive current flow, due for example to an output load short-circuit.
A typical solution provides for a negative feedback loop, which comprises a current sensing MOSFET, whose drain and gate are respectively connected to the drain and the gate of the main power MOSFET, and whose source is connected, through a current sensing resistor, to the source terminal of the integrated component. The base-emitter junction of a bipolar junction transistor (BJT) is connected across said sensing resistor, while the collector of the BJT is connected to the common gate of the two MOSFETs. A gate resistor is finally connected in series between an external gate terminal of the integrated component and the common gate.
In practical implementations, both the sensing MOSFET and the main power MOSFET are made up of similar cells, even if the latter comprises a much larger number of such cells than the former.
The current flowing through the sensing MOSFET, which is a small fraction of the current flowing through the main power MOSFET, generates a voltage drop across the sensing resistor whose value is chosen in such a way that when the current flowing through the power MOSFET exceeds a prescribed maximum value, the fractional current flowing through the sensing resistor generates a voltage drop sufficient to turn on the BJT. By draining current from the gate resistor through the BJT, the voltage applied to the gate of the two MOSFETs decreases, thus preventing the current flowing through them from further increasing.
Furthermore, the positive thermal coefficient of such feedback loop improves the protecting performances of this protection circuit, since at higher temperature the maximum value for the current decreases.
In the International Application No. WO 91/09424, a power device is described which includes in the same substrate as the power transistor a feedback loop of the type described above. The sensing resistor is made up of a polysilicon strip obtained simultaneously with the polysilicon gate layer of the MOS cells.
This solution requires a dedicated area on the substrate surface to allocate the sensing resistor, which is external to the sequence of source cells.